I just bought 300 dollars worth of courses couple days ago on Udemy, I've used all these companies and I have to say Udemy is better than them. There is definitely a market based on teaching quality difference and the teaching tools provided.
And yes, most of the courses that I have taken there are taught by professionals with industry experience.
You're right that Coursera has content from universities, on the other hand Udemy has individual instructors (and anyone can become an instructor). Also Coursera has a lot of free content - not sure if 100%, but a large number of courses offers the content for free, you only need to pay if you want to do assignments and get a certificate.
Oh, and Coursera does not have 90% off promos every other week, unlike Udemy